1. Field of the Invention
The present invention relates to a fiber optic communication system which uses wavelength division multiplexing to transmit a wavelength-multiplexed optical signal. More specifically, the present invention relates to a controller which controls an optical attenuator or an optical amplifier to change the power level of the wavelength-multiplexed optical signal when the number of channels are varied.
2. Description of the Related Art
Wavelength division multiplexing is used in fiber optic communication systems to transfer a relatively large amount of data at a high speed.
FIG. 1 is a diagram illustrating a conventional fiber optic communication system which uses wavelength division multiplexing to transmit, for example, four channels through a single optical fiber. Referring now to FIG. 1, transmitting units 20-1, 20-2, 20-3 and 20-4 transmit individual carriers having wavelengths .lambda.1-.lambda.4, respectively. Each carrier is modulated with information and represents an individual channel. The different carriers are multiplexed together by an optical multiplexer 22 into a wavelength-multiplexed optical signal. The wavelength-multiplexed optical signal is transmitted through an optical fiber 24 to an optical demultiplexer 26. Optical demultiplexer 26 branches the wavelength-multiplexed optical signal into four separate optical signals having the wavelengths .lambda.1-.lambda.4, respectively. The four separate branched optical signals are then detected by receiving units 28-1, 28-2, 28-3 and 28-4, respectively.
While the above optical fiber communication system multiplexes four carriers together, it is common practice to multiplex more than four carriers. More specifically, many different carriers may be multiplexed together. In this manner, a relatively large amount of data can be transmitted through an optical fiber.
An optical amplifier (not illustrated) or an optical repeater (not illustrated) is typically inserted between optical multiplexer 22 and optical demultiplexer 26, to amplify the wavelength-multiplexed optical signal travelling through optical fiber 24. Such an optical amplifier is typically a rare-earth doped optical fiber amplifier which directly amplifies the wavelength-multiplexed optical signal. That is, a rare-earth doped optical fiber amplifier amplifies the wavelength-multiplexed optical signal without converting the wavelength-multiplexed optical signal into an electrical signal.
Unfortunately, the use of a rare-earth doped optical fiber amplifier causes several problems when the number of channels in the wavelength-multiplexed optical signal is varied. More specifically, during the variation (that is, before the variation in the number of channels is complete), the optical power of each channel can undesirably be varied, thereby causing non-linear degradation or S/N degradation of the wavelength-multiplexed optical signal.